1. Field of the Invention
The present invention relates to a method for injection molding metallic materials to obtain a product of desired configuration by supplying a metallic material contained in an injection cylinder into a mold cavity.
2. Description of the Related Art
A typical example of known disk brakes for motor vehicles is shown in cross section in FIG. 32 hereof. As shown in FIG. 32, a disk rotor 503 of the example disk brake 502 is mounted to a drive shaft 500 via a hub 501. A peripheral portion of the disk rotor 503 is placed in a caliper 506.
A wheel 509 is braked by transmitting an oil pressure from a feed flow path 508 to a cylinder, not shown, of the caliper 506 and pressing brake pads 507, 507 onto a disk portion 505 of the disk rotor 503.
Since the brake pads 507, 507 are pressed hard onto the disk portion 505 of the disk rotor 503, the disk rotor 503 is required to be formed of a high-strength material.
On the other hand, in order to reduce the total weight of a motor vehicle, the disk rotor 503 is preferably manufactured of a lightweight material.
Metal matrix composite material (MMC) materials are well known for their high strength and lightweight. For example, employing aluminum (Al) alloy as a metal matrix can save the weight, and adding silicon carbide (SiC) to Al alloy matrix can realize increase in strength.
A method for manufacturing the disk rotor 503 from an Al alloy matrix composite material will now be described with reference to FIG. 34 hereof.
Such a method is carried out in a conventional apparatus for injection molding a metal matrix composite material. An injection cylinder 515 is brought into communication with a cavity 512 defined by a movable die 510 and a stationary die 511 via a gate 513. A plunger 516 is mounted to the injection cylinder 515 so as to be capable of upward and downward movements, an MMC feeding means 518 is connected to the injection cylinder 515 via a feed path 517, and a shut-off valve 519 is mounted to an exit side of the feed path 517.
A shut-off valve 519 is opened and an AL alloy matrix composite material is fed from the MMC feeding means 518 into the injection cylinder 515 as shown by arrow a. The plunger 516 is moved upward as shown by arrow b, and Al alloy matrix composite material is filled into the cavity 512 through the gate 513. Then, the movable die 510 is moved upward as shown by the arrow c to open the die and the cast product is taken out of the die. The cast product taken out will be described below.
Reference is made next to FIG. 34 showing a cast product taken out from the die. The cast product 520 is cut into a product portion 521 and a non-product portion 522.
The product portion 521 is a member formed of Al alloy matrix composite material molded in the cavity 512 and is to be processed to obtain the disk rotor 503 as shown in FIG. 32.
The non-product portion 522 is a member formed of Al alloy matrix composite material remained at the gate 513 (See FIG. 33).
The non-product portion 522 remained at the gate 513 is also of Al alloy matrix composite material obtained by adding SiC grains to Al alloy matrix. Therefore, since Al alloy matrix composite material cannot be reused as it is, it is necessary to separate SiC grains from Al alloy matrix in order to reuse it. However, such separation is technically difficult, and if possible, it costs much. Therefore, the non-product portion 522 is put on a shelf in the existing circumstances and this contributes to increase in the cost of the product portion 521 molded by Al alloy matrix composite material (or a metallic material).
On the other hand, there are products that do not require high strength among injection molded articles. Since such products are not required to contain SiC grains for increasing strength, they may be formed of normal aluminum alloy material (or a metallic material) by injection molding. Therefore, there is a tendency that they are considered to be easily reusable because it is not necessary to separate SiC grains as in the case of Al alloy matrix composite material when reusing the non-product remained at the gate.
However, in order to reuse the non-product portion as a molten material in subsequent injection molding, it is necessary to melt the non-product portion, which requires much heat energy for melting the non-product portion. Therefore, in the existing circumstances, the non-product portion is shelved, thereby contributing to increase in cost of the product molded of Al alloy material (metallic material).
FIG. 35 shows a conventional injection molding apparatus. The injection molding apparatus 600 includes an injection apparatus 601 opposed to a die 602. A molten metallic material 605 is poured into a cavity 604 through a gate 603. When the molten material 605 is solidified in the cavity 604, a disk rotor 608 for the disk brake is obtained.
FIG. 36 shows an example disk rotor including casting defects. The disk rotor 608 is a defective apparatus having a defect that is appeared on an opposite side of the gate 603 (See FIG. 35) as a boundary of imperfect integrity at the joint due to lowering of the temperature of the molten material, that is, cold shuts 606 and scabs 607. In other words, referring to FIG. 35, when the molten material 605 is injected into the cavity 604 through the gate 603 upwardly at the lower portion and then the molten material 605 collides against the upper portion of the cavity 604, the cold shuts 606 and scabs 607 are generated.
Referring now to FIG. 37A to FIG. 37C, a conventional injection apparatus will be described.
In FIG. 37A, a molten metal matrix composite material 703 is fed from the molten material feeding apparatus 704 into the injection cylinder 702 of the injecting apparatus 701.
In FIG. 37B, the injection cylinder 702 is connected to the forming die 705. The molten material 703 is injected into the cavity of the forming die 705 by the plunger 706 moving up and down in the injection cylinder 702, as shown by the arrow.
In FIG. 37C, the plunger 706 moves to the upper limit and injecting operation terminates. After that, when the injected molten material 703 is solidified in the forming die 705, and the cast product 707 is completed.
However, slugs 708 may enter into the cast product 707, which is a defect of the cast product. The slug 708 is a residue 709 generated by the oxide of the molten material 703 of metal matrix composite material, and is generated on the surface layer portion 711 of the molten material 703 and may adhere to the inner wall surface 712 of the injection cylinder 702. The adhered residue 709 causes a defect on the cast product 707 by being mixed in the molten material 703. When the defect is developed, the cast product has to be disposed, which leads to lowering of manufacturing efficiency.
It is therefore a first object of the present invention to enable the reuse of the non-product portion remained at the gate to bring down costs for injection-molded articles formed of metallic material.
A second object of the present invention is to provide an injection apparatus for metal matrix composite material that can eliminate defects of cast product caused by residues.
According to a first aspect of the present invention, there is provided a method for injection molding a metallic material in which an injecting material comprised of a half-solidified metallic material or a molten metallic material is injected into a cavity of a die from an injection cylinder through a gate thereof, the method comprising the steps of: taking out a cast product from the die while the cast product is still hot, the cast product having a product portion molded in the cavity and a non-product portion remaining at the gate; separating the non-product portion from the cast product while the non-product portion is in a state of high temperature; shaping the high-temperature non-product portion into a billet; putting the billet into the injection cylinder; and filling the injecting material into the injection cylinder to cause the billet to melt into the injecting material to thereby ready the billet for a succeeding injection cycle.
In other words, in the first invention, the non-product portion is shaped into a billet of high temperature, and the billet is melted in the injection cylinder by the use of heat from the injecting material. By utilizing heat from the injecting material for melting the billet, heat energy required to melt the billet can be reduced. In addition, since the billet is melted while it is in the state of high temperature, heat energy can further be saved. Therefore, the non-product portion remained at the gate can be reused without laying the non-product portion on the shelf.
Preferably, the step of taking out is carried out while the cast product is held at a temperature of 400 to 100xc2x0 C. In this temperature range, the cast product is half-solidified sot that it can be taken out from the die easily, but has heat that can be utilized as heat energy in melting the billet.
As an injecting material, for example, a metallic material in a half-solidified state and a metal matrix composite material in a molten state are prepared. The respective materials are filled into the injection cylinder such that the metallic material comes to the plunger side of the injection cylinder, and the metal matrix composite material comes to the gate side, so that they can be poured into the cavity in the order of the metal matrix composite material and the metallic material.
It is preferable to inject materials into the cavity in the sequence of the metal matrix composite material and the metallic material, because the metal matrix composite material is filled into the cavity and the metallic material remains at the gate. It is thus not necessary to separate a reinforcing material such as SiC grains from the metal matrix composite material when reusing the non-product portion remained at the gate.
Desirably, the high-temperature non-product portion is placed in the injection cylinder and the non-product portion is press-formed into a billet in the injection cylinder to thereby reduce expenses in pressurizing equipment by effective use of the injection cylinder and the plunger.
In addition, the inventors have found through researches to prevent generation of cold shuts and scabs that such cold shuts and scabs are generated mainly by the fact that the molten material gets cooled at the portion of the cavity remote from the entrance. Therefore, in order to make the molten material resist getting cooled, the inventors have succeeded in obtaining robust cast products for disk brakes by increasing the volume of the molten material contained in the part of the cavity remote from the entrance, and maintaining the temperature of the molten material flowing at the distance almost constant.
According to a second aspect of the present invention, there is provided an apparatus for molding a disk rotor including a disk-shaped brake ring portion, a cylindrical hub portion formed integrally with the brake ring portion and projecting a predetermined distance in one sideward direction, and a lid portion formed integrally with a top end of the hub portion, the apparatus comprising: a forming die including a stationary die and a movable die defining a cavity therebetween, the forming die being positioned such that that portion of the cavity for forming the brake ring portion and that portion of the cavity for forming the lid portion are arranged vertically; and the cavity portion for forming the brake ring portion having an overflow portion for increasing a volume of the cavity on an upper part thereof so that when a molten metal matrix composite material is poured into the cavity upwardly from below, the molten composite material flows into the overflow portion past said cavity portion.
In this molding apparatus, the molten metal matrix composite material is injected upwardly from below. Therefore, in the case of cavities for molding disk rotors, the molten material goes into the die from below, separates once to the left and the right, and joins again at the upper portion. Since the cavity has the overflow portion formed at the upper portion thereof for the brake ring portion, the volume of the cavity is increased by the overflow portion, and thus the amount of the molten material at the upper portion increases. As a consequence, the temperature of the top end of the molten material resists lowering, and thus cold shuts and scabs on the cast product of a disc rotor can be prevented from occurring when they are joined at the upper portion.
In a preferred form, the overflow portion is provided in opposed relation to one surface of the brake ring portion. This causes the volume of the overflow portion to be increased so that the molten material can easily be forced into the cavity.
The overflow portion may comprise one or more recesses extending radially from a center of the brake ring portion in the form of a groove. As a result, the inlet port of the overflow portion can be formed between the center side and the outer edge, and thus the volume of the contained molten material may be increased while reducing resistance applied when flowing into the overflow portion.
According to a third aspect of the present invention, there is provided an injection apparatus including a forming die into which a molten metal matrix composite material is poured upwardly from below, the apparatus comprising: an injection cylinder provided vertically; a plunger disposed vertically movably within the injection; a block extending upwardly from a top end of the plunger and having an outer diameter smaller than an inner diameter of the injection cylinder, the injection cylinder having an inner wall surface defining, jointly with an outer peripheral surface of the block, a clearance for accommodating a residue of the molten metal matrix composite material.
In this arrangement, since the block is not brought into contact with the residue attached on the inner wall of the injection cylinder, the block does not scrape off the residue. Therefore, when injecting the molten material, the block can push out the central portion of the molten material that is free of the residue ahead of other portions, and thus the residue can be prevented from getting mixed into the molten material.
The block may be detachably secured to the plunger. More specifically, the plunger may be formed with a projection on the head portion thereof, while the block is formed with a recess on the lower surface thereof, so that the projection of the plunger can be detachably fitted in the recess of the block. As a result, the positioning and mounting of the block with respect to the plunger do not take a lot of trouble.
It is desirable that the block is formed of a material harder than the metal matrix composite material after it is solidified. In this arrangement, even when an impact is applied by a hammer or the like to the portion of the cast product corresponding to the gate in order to take out the block adhered to the portion of the solidified cast product corresponding to the gate after injection is terminated, the block in the cast product is free from deformation and scratches, whereby the block can be reused.